Conventional battery chargers used to charge rechargeable batteries such as Nickel-Cadmium (NiCd) or Nickel Metal Hydride (NiMH) batteries in a rapid fashion typically drop the rate at which the battery is being charged once the battery begins to increase in temperature. For example, a typical rapid charger used to charge two-way radio batteries begins to charge the battery at a charge rate approximating the capacity of the battery in one hour (charge rate of "C"/hr where "C" is the capacity of the battery), the battery charger then drops the charge current to a trickle rate (e.g., C/20 or less) once the battery starts to increase in temperature by a certain amount over time (reaches a certain .DELTA.T/.DELTA.t). Usually the battery is between 88% and 93% charged at the time the charger switches to the trickle charge. At the trickle charge rate, the battery becomes fully charged after 10 or more hours in the battery charger.
The main reason for dropping the charge rate from a rapid rate to a trickle charge rate is to avoid excessive heating of the battery cells, since the charge efficiency of the battery cells decreases as the battery temperature increases. However, although reducing the charge rate protects the battery from thermal damage, the trickle charge rate requires 10 hours or more to fully charge the battery. A need thus exists for a method and apparatus which can fully charge a rechargeable battery faster but yet protect the battery cells from excessive heating.